Since the accumulation of ice on the various surfaces of an aircraft can produce disastrous results, it is important for the pilot to know when ice starts to appear and how thick the layer is. This knowledge allows the pilot to take measures to remove the ice, such as turning on surface heaters or changing the flight speed or elevation. While various attempts have been made to produce ice detectors, they have been limited by their accuracy, by their inability to distinguish between ice and water, and by their inability to measure the thickness of the ice.
One such prior art device shown in U.S. Pat. No. 3,247,478 includes two electrodes shaped like interleaved comb-like grids. When the relative humidity changes, the resistance of the device changes. Thus, by measuring the resistance, the relative humidity may be determined. However, this device does not measure the thickness of ice, nor does it distinguish between ice and water.
In U.S. Pat. No. 3,422,677, a device is shown where ice is detected by having a combination of a humidity detecting circuit and a temperature measuring circuit. When the device senses the presence of moisture and a temperature below the freezing point the presence of ice is indicated. When ice is detected, a heater may be actuated to remove it. While this device is designed to detect the presence of ice, it does not allow for a measurement of the thickness of the ice. It also requires several different temperature transducers in order to detect the ice.
Other previous methods used to detect ice and determine its thickness include visual observation, ultrasonic vibrating sensing probes and probes with optical sensors. These methods are not very accurate in either detection or thickness determination. Also, visual observation can only be used for detection and is severely limited by darkness or other poor visibility conditions. The other methods using probes are complex and the protruding probes cause undesirable drag.
Accordingly, it is an object of this invention to provide a device which accurately detects the presence of ice on a surface.
It is a further object of this invention to provide a device which accurately determines the thickness of ice on a surface.
It is a further object of this invention to accomplish the foregoing objects simply and cheaply.
It is a further object of this invention to accomplish the foregoing objects without causing undesirable drag as the surface moves through a viscous medium.
It is a further object of the invention to accomplish the foregoing objects independently of darkness or other poor visibility conditions.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings which follow.